We just got done forming a foundation wall that’s going to be supporting 2×6 walls directly on them without a deck. The designer specified 8″ thick foundation walls with a ledger near the deck. The ledger reduces the wall down to 6″ wide above the deck.
To simplify the foundation wall pour we had them install 2 inches of foam board in the top of their panels. The best part is the foundation needs to be insulated with 2 inches of foam board prior to backfilling, so this piece of foam can stay on the walls after the forms are poured.
Foam board is great for creating all kinds of block-outs in foundations. It’s super easy to cut and install in the formwork and it’s strong enough so that it won’t crush under the load of the wet concrete.